When manufacturing an LSI, tungsten is widely used for a MOSFET gate electrode, a contact between a source and a drain, a word line of a memory and the like. Copper wiring is mainly used in a multilayer wiring process. However, copper is poor in heat resistance and is prone to diffuse. Thus, tungsten is used in a portion where a heat resistance is required or a portion where deterioration of electric characteristics due to copper diffusion is concerned.
A physical vapor deposition (PVD) method has been used as a tungsten film forming process in the past. However, it is difficult for the PVD method to cope with a portion where high coverage (step coverage) is required. Therefore, a film is formed by a chemical vapor deposition (CVD) method with good step coverage.
As a method of forming a tungsten film (CVD-tungsten film) by the CVD method, there is generally used a method of generating a reaction of WF6+3H2→W+6HF on a semiconductor wafer as a target substrate, using, for example, a tungsten hexafluoride (WF6) gas as a raw material gas and a H2 gas as a reducing gas.
However, in the case of forming a tungsten film by CVD using a WF6 gas, there has been a strong concern that fluorine included in WF6 reduces a gate insulating film and deteriorates electric characteristics thereof, particularly in a gate electrode, a word line of a memory or the like of a semiconductor device.
Tungsten hexachloride (WCl6) is known as a raw material gas for the formation of a CVD-W film not containing fluorine. Similar to fluorine, chlorine has a reducing property. However, the reactivity of chlorine is weaker than that of fluorine. It is expected that chlorine has a reduced adverse effect on electric characteristics.
Recently, the miniaturization of semiconductor devices is progressing. Even with CVD which is said to be capable of achieving good step coverage, it is difficult to perform the embedding into a complex shape pattern. From the viewpoint of obtaining even higher step coverage, attention is paid to an atomic layer deposition (ALD) method in which a raw material gas and a reducing gas are sequentially supplied while performing a purge process in the course of sequentially supplying the raw material gas and the reducing gas.
On the other hand, in the case of forming a tungsten film by CVD or ALD, the adhesion to an oxide film such as an interlayer insulating film or the like is poor and the incubation time is prolonged. Therefore, film formation is difficult. For this reason, a Ti-based material film such as a TiN film is used as a base film.
However, a tungsten chloride gas used as a tungsten raw material has a property of etching a material that constitutes a base film such as a TiN film. When forming a tungsten film, a base film such as a TiN film is etched. Thus, there is a possibility that the thickness of the tungsten film becomes smaller than a required film thickness.